An actuator effectively generates power. Especially, the actuator of a fluid driving system is widely used. This actuator is driven by pressure changes in the fluid. Well known examples include oil hydraulic actuators and pneumatic actuators. The structure of these actuators is simple. In addition to this benefit, the actuators are lightweight and inexpensive. Therefore, this kind of actuator is used in piston cylinders and a vane motor.
However, in order to effectively lead the power from the actuator, a seal for the activated fluid is necessary as a structure of the actuator. The seal is a packing between the cylinder and the piston. Friction generated in the actuator is large when the actuator is activated by a pressure change of the fluid. Accordingly, it is impossible to identify the location of the actuator with high accuracy.
For example, FIG. 1 is a horizontal sectional plan of a pneumatic actuator according to the prior art. The pneumatic actuator includes a piston, a cylinder and a plurality of seals connected between the piston and the cylinder. When fluid A is supplied to the cylinder, the piston moves to position A of the cylinder along move direction A. When fluid B is supplied to the cylinder, the piston moves to position B of the cylinder along move direction B. In short, by changing the supply of the fluid A and the fluid B, the piston is moved between position A and position B. Therefore, for example, if the piston is connected to an automatic door of a bus, the opening and closing of the automatic door is controlled by changing position A to position B or vice versa A. However, in this pneumatic actuator, a plurality of seals leads big friction between the piston and the cylinder. Therefore, even if the piston moves between the position A and the position B of the cylinder, only two positions (position A and position B) can be used to generate automatic power for other equipment (the automatic door). In other words, it is impossible to identify with high accuracy and use a large number of positions to generate automatic power.